Telecommunication systems are well known in the art. From the seminal work of Samuel Morse, U.S. Pat. No. 1,647 (1840) and Alexander Graham Bell, U.S. Pat. No. 174,465 (1876), an entire industry of telecommunications has developed spanning the globe and beyond.
The encoding of messages as well as timing has played a key role throughout the historical development of communication systems. For example, prior to the invention of the telephone, messages were encoded into Morse Code and corresponding electronic pulses transmitted the encoded message over telegraph lines which would then be received and decoded. Thereafter a response could be communicated reversing the operations. Manual encoding and decoding of messages prohibited the direct real time communications between two persons.
With the advent of the telephone, real time communication was made possible through the electronic encoding of voice patterns into communication signals which signals were carried over wires between two telephones. The speed of the electronic communication signal, which far exceeded the speed of sound, permitted real time voice communication between individuals at substantial distances without significantly perceptible time delay.
Today communication signals are not constrained to wires but are also carried by a microwave, radiowave and optic fibers. These advances have permitted global real time telecommunications. Moreover, real time communication service is expected by the consuming public.
Unlike conventional hard wired telephone systems where a single telephone communication signal is carried on a pair of wires, time division multiplexing has been utilized to increase the capacity of the various carrier mediums. For example, many communication signals can be multiplexed together and carried over a single optic fiber. Accordingly a single optic fiber cable can replace a hundred pair wire cable and provide even greater signal carrying capacity.
The same principle has been employed with respect to radio telephone systems. Radio telephone systems for both stationary and mobile uses are well know in the art. For example, in remote rural areas where installation and maintenance of conventional telephone wire lines is prohibitive, radio telephone systems permit the broadcast between a base and sundry subscriber stations to facilitate telephone service. Mobile radio telephone systems are also becoming increasingly more prevalent in the form of the cellular car phones which have become widely available.
Radio telephone systems utilize a group of selected radio frequencies for carrying the communication signals in lieu of wire cables. A typical stationery radio telephone system may include 13 pairs of selected frequencies or channels over which communication signals are broadcast and received between subscriber stations and a common base station.
Due to the fact that only a limited band of frequencies is permitted for radio telephone usage, time division multiplexing has been employed to permit increased capacity of radio telecommunication systems. For example, U.S. Pat. No. 4,675,863 discloses a stationary radio telephone system which utilizes 26 channel pairs each of which can carry up to four communication signals at one time.
Unlike fiber optic communication transmissions which can speed communication signals to and from their destination in the gigahertz range, carrier radio frequencies (channels) are significantly more limited in their capacity.
In order to increase the capacity of the radio channels, voice signal compression techniques have been utilized. One technique which has proved successful is Residual Excited Linear Predictive coding (RELP) such as disclosed in U.S. patent application No. 667,446, filed Nov. 2, 1984. RELP permits the compression of a 64 kilobits per second voice communication signal into an 14.6 kilobits per second encoded signal which is transmitted over the radio channel. The 14.6 kilobit per second is decoded when it is received to reconstruct a 64 kilobits per second signal with virtually no perceptible loss in signal quality.
Underlying the mechanics of RELP is a recursive encoding and decoding formulation which relies upon the harmonics of the human voice which provide statistically predictable wave patterns. Unlike voice transmissions, however, data communication signals, such as modem and fax (telecopier) signals, do not exhibit the harmonic qualities which are characteristic of voice signals. Accordingly, the RELP signal compression technique which is employed for voice signals is not entirely suitable for fax and modem communication signals. It would be desirable to provide a more suitable coding compression system for data signals.